In range imaging, each pixel of a two-dimensional image of a three-dimensional scenery is associated with information about a distance from an imaging point to a point in the scenery imaged by that pixel. In other words, each pixel includes a distance-to-object information in addition to the brightness and/or color information. Range imaging is used in applications where distance to and/or three-dimensional shape of objects being imaged needs to be determined, such as 3D profiling, gaming, human-to-computer interface, vision through fog, automotive, machine vision, etc.
Range imaging is based on measuring, for every pixel of the image, of the time that it takes light to propagate from a light source to the object and back to the detector. One type of a range imaging system is similar to radar. A narrow-beam laser pulse is emitted towards an object, and the strength (optical power) of the laser pulse reflected from a part of the object illuminated by the pulse is measured. The time delay between the emitted pulse and the reflected pulse is also measured. The laser beam is steered in a raster fashion, and the measurements are repeated for each angle of the beam. In this way, a raster image is obtained, in which every pixel is associated with a time delay, which corresponds to amount of time that took the emitted laser pulse to propagate towards the object and back towards the detector. The amount of time is proportional to a distance to the part of the object illuminated with the laser beam.
A more efficient approach utilizes a laser source whose output is spatially modified to form a particular pattern of dots on the object. The reflected pattern is detected and the spatial information is decoded via triangulation to detect the position of the object.
A faster approach to range imaging consists in using so called gated detector arrays. A single broad-beam light pulse illuminates the entire scenery to be imaged. The detector array is “gated”, that is, it is equipped with an electronic gate or shutter that makes the detector array responsive to light only during a narrow time window when the “gate” is open. The moment of opening the “gate” is delayed by a delay time with respect to the moment the light is emitted. The emitted light propagates a pre-defined distance corresponding to the delay time, reflects from an object located at that distance, and propagates back. Any light reflected from an object located before or after the pre-defined distance will be suppressed by the gated detector array. The time delay is varied to obtain 3D imagery slice-by-slice.
A gated detector array approach can be useful for imaging through fog, for example. Light reflections from a fog patch, which would normally obscure objects located beyond the fog patch, can be suppressed by setting the pre-defined distance to be larger than the fog patch thickness.
Yet another approach to range imaging consists in modulating the illuminating light at a high (radio) frequency and detecting, for each pixel of a detector array, a modulation phase delay between the illuminating light and light detected by the pixel. The modulation phase delay is proportional to a distance from the imaging point to a point in the scenery imaged by the pixel. One advantage of this approach is that the need in high peak intensity pulsed light sources is eliminated, which, however, comes at a cost of reduced sensed distance range due to aliasing effects. The reduced distance range is perhaps of less importance in gaming or gesture recognition applications, where the user is typically positioned within a known and relatively narrow range of distances from the gaming console or a computer system.
Cameras based on gated detector arrays or on detector arrays sensitive to modulation phase have been termed “time-of-flight cameras” because they are sensitive to time it takes the illuminating light to propagate towards the object and back to the camera. Time-of-flight cameras began to emerge when semiconductor devices became fast enough to process images at practical frame rates of tens of frames per second.
In any of the above range imaging methods, strict requirements are imposed on illuminating light sources. They have to be fast, powerful, and reliable. Modulation frequency is typically in the order of 100 MHz, with rise/fall times on the order of 1 ns to achieve the desired accuracy. Laser diodes and high-brightness light-emitting diodes (LEDs) are frequently used for this application at power levels of hundreds of milliwatts to Watts to achieve distances that allow computer or living room operation, respectively. Infrared laser diodes and LEDs are employed to avoid dazzling users with bright light.
The reliability and lifetime requirements for laser diodes are particularly difficult to meet. For gaming and computer interface/gesture recognition applications, the illumination levels have to remain relatively unchanged for tens of thousands hours of operation. At present, very few, if any, laser diodes or LEDs can provide Watt-level illumination with adequate reliability. When the illumination levels decrease due to aging of the laser diodes and/or LEDs, time-of-flight cameras cease to operate with the desired level of performance.
An object of the present invention is to provide an illumination source reliable enough for use in range imaging systems, and in particular in time-of-flight camera systems.